Harju County (Estonian: Harju maakond), or Harjumaa (German: Harrien, Latin: Harria) is one of the 15 counties of Estonia. It is situated in Northern Estonia, on the southern coast of the Gulf of Finland, and borders Lääne-Viru County to the east, Järva County to the south-east, Rapla County to the south, and Lääne County to the south-west. In January 2015 Harju County had a population of 575,601 – constituting 43.8% of the total population of Estonia. Tallinn, the capital and largest city of Estonia, constitutes about 72% of the county's population with its 413,782 inhabitants.
The territory of the modern Harju County consisted mostly of two ancient Estonian counties: Revala, around what is now Tallinn, and Harjumaa, which was situated south of Revala and is today situated mostly in Rapla County. Lindanise, then a small trading post at the Gulf of Finland, served as the capital of Revala. It eventually grew into the mostly German-populated Hanseatic town of Reval and later into the Estonian capital Tallinn.
Aru is a village in Kuusalu Parish, Harju County in northern Estonia.
Coordinates: 59°25′29″N 25°29′25″E / 59.42472°N 25.49028°E / 59.42472; 25.49028